Once you have all parts downloaded and sitting in one folder:
: If the extraction stops and asks for "part06," make sure the file is named exactly like the others (e.g., Just_Cause_4_Complete_Edition-Razor1911.part06.rar ) and sits in the same directory as Part 05 and Part 07.
Are you getting a when you try to extract that file?
: You only need to right-click on Part 01 and select "Extract Here." The software (like WinRAR or 7-Zip) will automatically pull data from Part 02, Part 03, and eventually your Part 06 to rebuild the full installer. Common Issues with Part 06